Get rid of double indirection

[marton78]

Hi, first of all thanks for this great library.

One hint: you could get rid of the double indirection in

using OnMessageCallback = std::function<void(const WebSocketMessagePtr&)>;

by changing it to

using OnMessageCallback = std::function<void(const WebSocketMessage&)>;

This could also be done in a backwards-compatible fashion by offering a second overload, e.g.:

using OnMessageCallbackNew = std::function<void(const WebSocketMessage&)>;
void setOnMessageCallback(const OnMessageCallbackNew& callback);